Peak-to-average power ratio management for multi-carrier modulation in wireless communication systems

ABSTRACT

Techniques for managing peak-to-average power ratio (PAPR) for multi-carrier modulation in wireless communication systems. Different terminals in a multiple-access system may have different required transmit powers. The number of carriers to allocate to each terminal is made dependent on its required transmit power. Terminals with higher required transmit powers may be allocated fewer carriers (associated with smaller PAPR) to allow the power amplifier to operate at higher power levels. Terminals with lower required transmit powers may be allocated more carriers (associated with higher PAPR) since the power amplifier is operated at lower power levels. The specific carriers to assign to the terminals may also be determined by their transmit power levels to reduce out-of-band emissions. Terminals with higher required transmit powers may be assigned with carriers near the middle of the operating band, and terminals with lower required transmit powers may be assigned with carriers near the band edges.

BACKGROUND

I. Field

The present invention relates generally to data communication, and morespecifically to techniques for managing peak-to-average power ratio(PAPR) for multi-carrier modulation in wireless communication systems.

II. Background

Wireless communication systems are widely deployed to provide varioustypes of communication such as voice, data, and so on. These systems maybe multiple-access systems capable of supporting communication withmultiple users by sharing the available system resources (e.g.,bandwidth and transmit power). Examples of such multiple-access systemsinclude code division multiple access (CDMA) systems, time divisionmultiple access (TDMA) systems, frequency division multiple access(FDMA) systems, and orthogonal frequency division multiple access(OFDMA) systems.

A wireless communication system may employ multi-carrier modulation fordata transmission. Common examples of multi-carrier modulation includeorthogonal frequency division multiplexing (OFDM) and discretemulti-tone (DMT). OFDM effectively partitions the overall systembandwidth into a number of orthogonal subbands. Each subband isassociated with a respective carrier upon which data may be modulated.The carriers for the subbands may be independently modulated with data,and the modulated carriers are then added together to generate an outputwaveform.

Multi-carrier modulation has certain desirable characteristics,including the ability to combat multipath effects. However, a majordrawback with multi-carrier modulation is high peak-to-average powerratio (PAPR) for the output waveform, i.e., the ratio of the peak powerto the average power of the waveform generated by multi-carriermodulation can be high. The high PAPR results from possible in-phase (orcoherent) addition of all the carriers when they are independentlymodulated with data. In fact, it can be shown that the peak power can beup to N times greater than the average power for multi-carriermodulation, where N is the number of carriers.

The high PAPR for the waveform generated by multi-carrier modulationnormally requires the power amplifier to be operated at an average powerlevel that is typically much lower than the peak power level (i.e.,backed off from peak power). This is because large peaks in the waveformmay cause the power amplifier to operate in a highly non-linear regionor possibly clip, which would then cause intermodulation distortion andother artifacts that can degrade signal quality. By operating the poweramplifier at a back-off from peak power, where the back-off typicallyranges from 4 to 7 dB, the power amplifier can handle large peaks in thewaveform without generating excessive distortion. However, the back-offrepresents inefficient operation of the power amplifier during othertimes when large peaks are not present in the waveform. Thus, it ishighly desirable to minimize the PAPR of the waveform so that the poweramplifier can be operated closer to the peak power level if desired ornecessary.

Various schemes have been introduced to minimize PAPR for multi-carriermodulation. Most of these schemes strive to reduce the PAPR of thewaveform itself. For example, one conventional scheme proposes mappingthe data to be transmitted into specific codewords that have beenspecially selected because they are associated with low PAPRs. Anotherconventional scheme proposes using “peak reduction carriers” that aremodulated in a manner to reduce peaks in the waveform. Yet anotherconventional scheme proposes modulating data on all carriers but withdifferent phases to attempt to reduce the PAPR of the waveform. Thesevarious conventional schemes for reducing PAPR may not be applicable forcertain multi-carrier communication systems. This may be the case, forexample, if the data for all carriers is not available or accessible, asdescribed below.

There is therefore a need in the art for techniques for managing PAPRfor multi-carrier modulation in wireless communication systems.

SUMMARY

Techniques are provided herein for managing PAPR in various wirelessmultiple-access multi-carrier communication systems (e.g., OFDMAsystems). It is recognized that different terminals in a multiple-accesscommunication system may be associated with different required transmitpowers to achieve their desired received signal qualities. Carriers maybe assigned to terminals based on their required transmit powers.

In an aspect, the number of carriers to allocate to each terminal ismade dependent on its required transmit power. Fewer carriers may beallocated to a terminal with a higher required transmit power. Since asmaller PAPR is associated with a waveform generated with fewercarriers, the power amplifier may be operated with a smaller back-offand the waveform may be transmitted at a higher power level. Conversely,more carriers may be allocated to a terminal with a lower requiredtransmit power. Even though a larger PAPR is associated with a waveformgenerated with more carriers, the power amplifier can provide the largerback-off since the required transmit power is lower.

In another aspect, the specific carriers to assign to the terminals aredetermined by their transmit power levels. Terminals with higherrequired transmit powers are more likely to generate high levels ofintermodulation distortion. These terminals may be assigned withcarriers near the middle of the operating band so that their distortionmay fall within the operating band. Conversely, terminals with lowerrequired transmit powers are likely to generate low levels ofintermodulation distortion. These terminals may be assigned withcarriers near the edges of the operating band since the distortion willlikely be below specified out-of-band emission requirements.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 shows a diagram of a wireless multiple-access communicationsystem;

FIG. 2 shows a subband/carrier structure that may be used for an OFDMAsystem;

FIG. 3A shows two hypothetical waveforms generated by multi-carriermodulation;

FIG. 3B shows transmission of the two waveforms with a maximum transmitpower of P_(max) and in a manner to minimize intermodulation distortion;

FIG. 3C shows transmission of the two waveforms, given the maximumtransmit power of P_(max) and using power control to attain the desiredreceived signal quality;

FIG. 4 shows assignment of carriers to terminals in a manner to reduceout-of-band emissions;

FIG. 5 shows a process to allocate and assign carriers to terminals; and

FIG. 6 shows a block diagram of an access point and two terminals.

DETAILED DESCRIPTION

FIG. 1 shows a diagram of a wireless multiple-access communicationsystem 100 that employs multi-carrier modulation. System 100 includes anumber of access points 110 that communicate with a number of terminals120 (only two access points 110 a and 110 b are shown in FIG. 1 forsimplicity). An access point is a fixed station that is used forcommunicating with the terminals. An access point may also be referredto as a base station or some other terminology.

A terminal is a station that communicates with the access point. Aterminal may also be referred to as an access terminal, a user terminal,a remote station, a mobile station, a wireless communication device, orsome other terminology. Each terminal may communicate with one ormultiple access points on the downlink and/or uplink at any givenmoment. The downlink (i.e., forward link) refers to transmission fromthe access point to the terminal, and the uplink (i.e., reverse link)refers to transmission from the terminal to the access point.

A system controller 130 couples to the access points and may furthercouple to other systems/networks (e.g., a packet data network). Systemcontroller 130 provides coordination and control for the access pointscoupled to it. Via the access points, system controller 130 furthercontrols the routing of data among the terminals, and between theterminals and other users coupled to the other systems/networks.

The techniques described herein for managing PAPR may be implemented invarious wireless multiple-access multi-carrier communication systems.For example, system 100 may be an OFDMA system that utilizes OFDM fordata transmission. Moreover, these techniques may be used for the uplinkas well as the downlink. For clarity, these techniques are describedspecifically for the uplink in an OFDMA system. In the followingdescription, an active terminal is one that is scheduled for datatransmission on the uplink (and possibly the downlink).

FIG. 2 shows a subband/carrier structure 200 that may be used for anOFDMA system. The system has an overall system bandwidth of W MHz, whichis partitioned into N orthogonal subbands 210 using OFDM. Each subbandhas a bandwidth of W/N MHz and is associated with a respective carrier212 upon which data may be modulated.

In a typical OFDM system, only M of the N total carriers are used fordata transmission, where M<N. The remaining N−M carriers are not usedfor data transmission and their associated subbands serve as guardsubbands to allow the system to meet spectral mask requirements. The Musable carriers include carriers F through F+M−1, where F is an integernormally selected such that the M usable carriers are centered in themiddle of the operating band.

For OFDM, up to N carriers for the N subbands may be independentlymodulated with data. The modulated carriers are then added together toform an output waveform. The modulated carriers may add coherently(i.e., in-phase), in which case there will be a large amplitude in thewaveform. It can be shown that the peak power of the waveform generatedwith N independently modulated carriers can be many times greater thanthe average power of the waveform. The exact value for PAPR depends onmany factors. Moreover, the value of interest is often not the absolutepeak value but some statistical value, e.g., what value of instantaneouspower is exceeded say 99% of the time.

FIG. 3A shows plots of two hypothetical waveforms 310 and 312 that aregenerated by multi-carrier modulation. The horizontal axis denotes timeand the vertical axis denotes power. Waveform 310 is generated with Lcarriers, and waveform 312 is generated with 2·L carriers, where L maybe any integer greater than one. The average power of waveform 310 isthe same as that of waveform 312. However, the peak power of waveform312 is twice that of waveform 310 because twice the number of carrierswas used to generate waveform 312. Consequently, the PAPR of waveform312 is larger than the PAPR of waveform 310.

A waveform generated by multi-carrier modulation is typicallytransmitted in a manner to limit the amount of intermodulationdistortion. This requires the power amplifier for the waveform to beoperated at an average power level, P_(avg), that is reduced or backedoff from the peak or maximum power level, P_(max), for the poweramplifier. The amount to back off is selected such that the poweramplifier does not (or minimally) operate in a highly non-linear regionor clip. More specifically, the back-off is normally selected such thatthe distortion generated by the power amplifier is limited to aparticular level.

FIG. 3B shows the transmission of the two waveforms in FIG. 3A with themaximum transmit power of P_(max) and in a manner to minimizedistortion. Waveform 310 may be transmitted with a back-off of BO₁,which is determined in part by the PAPR₁ for this waveform (e.g.,BO₁≦PAPR₁). Similarly, waveform 312 may be transmitted with a back-offof BO₂, which is determined in part by the PAPR₂ for this waveform(e.g., BO₂≦PAPR₂). The average transmit power (P_(avg1)) of waveform 310may be approximately twice the average transmit power (P_(avg2)) ofwaveform 312 while still limiting the distortion to approximately thesame level. The exact ratio of P_(avg1) to P_(avg2) is dependent on thespecific back-offs used for waveforms 310 and 312.

For an OFDMA system, the M usable carriers may be shared among multipleactive terminals. On the uplink, each active terminal may be allocated aspecific set of carriers upon which it may transmit data. The number ofcarriers to allocate to each active terminal and which specific carriersto assign to the terminal may both be determined as described below. Thecarriers assigned to each terminal may or may not be contiguous. Eachactive terminal may then transmit using its specific assigned carriers.

Referring back to FIG. 1, the terminals may be dispersed throughout thesystem. Each terminal is associated with a particular path loss to itsaccess point, which is largely dependent on the distance between theterminal and the access point. Each terminal also requires a particularreceived signal quality at the access point to achieve a target level ofperformance. The required received signal quality may be quantified by aparticular received signal-to-noise ratio (SNR), and the target level ofperformance may be quantified by a particular frame error rate (FER),packet error rate (PER), and so on. The required transmit power for eachterminal is dependent on its path loss and its required received signalquality.

If the terminals are dispersed throughout the system, then the path lossis typically different from terminal to terminal. Moreover, the desiredreceived signal quality may be different from terminal to terminaldepending, for example, on their data rates. Thus, the required transmitpower is typically different from terminal to terminal. In general,terminals that are located farther away from the access point havegreater path losses to the access point and would then require highertransmit powers to achieve a given received signal quality. For example,terminals 120 a, 120 b, 120 d, and 120 g will likely require moretransmit power than terminals 120 c, 120 e, and 120 f to achieve thesame received signal quality at their respective access points.

Each terminal is associated with a particular maximum transmit power,P_(max), that may be used for data transmission. This maximum transmitpower may be determined by regulatory constraints, system design, and/orlimitations of the power amplifier used by the terminal. The maximumamount of transmit power that may be used for uplink data transmissionwould then be limited to P_(max).

A power control loop may be maintained to control the transmit power ofeach active terminal. Because a large disparity may exist in the pathlosses for the active terminals, the received powers at the access pointfor these terminals may vary by a large amount (e.g., by as much as 80dB) if these terminals all transmit at the same power level. Even thoughorthogonal subbands are generated by OFDM, the uplink transmissions fromthe active terminals may interfere with one other due to, for example,offsets in their timing and/or frequency. To limit the amount ofinterference to nearby carriers, the transmit power of each activeterminal may be controlled or adjusted such that the received signalquality for the terminal is within an acceptable range. The requiredtransmit power for each terminal would then be determined based on theuplink power control, which may be coarse.

In an aspect, the number of carriers to allocate to each active terminalis dependent on its required transmit power. Thus, different numbers ofcarriers may be allocated to different terminals depending on theirrequired transmit powers. Higher transmit power is required to achievethe desired received signal quality when the path loss is greater. Ifhigher transmit power is required, then fewer carriers may be allocated.Since a smaller PAPR is associated with a waveform generated with fewercarriers, the power amplifier may be operated with a smaller back-offand the waveform may be transmitted at higher power level. Conversely,since lower transmit power is required when the path loss is smaller,more carriers may be allocated. Even though a larger PAPR is associatedwith a waveform generated with more carriers, the power amplifier canprovide the larger back-off since the required transmit power for thewaveform is lower.

FIG. 3C shows the transmission of the two waveforms in FIG. 3A, giventhe maximum transmit power of P_(max) and using power control to attainthe desired received signal quality. Waveform 310 is transmitted with arequired average power of P_(req1), which is backed off by at least BO₁from P_(max). Waveform 312 is transmitted with a required average powerof P_(req2), which is backed off by at least BO₂ from P_(max). Therequired average powers of P_(req1) and P_(req2) may be determined bythe path losses and the required received signal qualities associatedwith the terminals transmitting these waveforms. The higher requiredaverage power for waveform 310 may be due to a greater path loss and/ora higher required received signal quality for the waveform. Theback-offs of BO₁ and BO₂ may be determined based on the PAPRs of thesewaveforms, as described above.

As shown in FIG. 3C, for a power amplifier constrained by the maximumtransmit power of P_(max), the higher required average power of P_(req1)for waveform 310 can be provided by the power amplifier since thiswaveform is generated with fewer carriers and is associated with asmaller back-off. Even though waveform 312 is generated with morecarriers and is associated with a larger back-off, the required averagepower of P_(req2) for this waveform can also be provided by the poweramplifier since this power level is lower.

The maximum number of carriers that may be allocated to each activeterminal may thus be made dependent on the required transmit power andthe maximum transmit power for the terminal. The determination of themaximum number of carriers that may be allocated to each terminal may bemade based on various schemes, two of which are described below.

In a first carrier allocation scheme, a table is formed for maximumallowed average power versus number of carriers. This table can includeone entry for each possible number of carriers that may be assigned. Forexample, the table may include N entries for N carriers, where i denotesthe number of carriers for the i-th entry in the table. For each entry,the highest average power P_(mavg,i) that may be used for the associatednumber of carriers, i, is determined (e.g., empirically, by simulation,or via some other means). This maximum allowed average power,P_(mavg,i), is based on an assumption of the maximum transmit power ofP_(max) for the terminals (which may be specified for the system or byregulatory constraints). The table may be formed as shown in TABLE 1.

TABLE 1 Number Maximum Allowed of Carriers Average Power N P_(mavg,N) .. . . . . i P_(mavg,i) . . . . . . 1 P_(mavg,1)Since waveforms with more carriers are associated with larger back-offs,the maximum allowed average power decreases with increasing number ofcarriers (i.e., P_(mavg,1) >P_(mavg,2)> . . . >P_(mavg,N))

The maximum number of carriers that may be allocated to each activeterminal may then be determined based on the required transmit power,P_(req), for the terminal and the table. In particular, the requiredtransmit power for the terminal may be compared against the maximumallowed average powers in the table. The smallest maximum allowedaverage power (P_(mavg,S)) that is higher than or equal to P_(req) isidentified, and the number of carriers S associated with this P_(mavg,S)is determined. The terminal may then be allocated any number of carriersless than or equal to S.

In a second carrier allocation scheme, a table is formed for requiredback-offs versus number of carriers. This table can also include oneentry for each possible number of carriers that may be assigned. Foreach entry, the minimum back-off BO₁ required for the associated numberof carriers, i, is determined (e.g., empirically, by simulation, or viasome other means). This table may be formed as shown in TABLE 2.

TABLE 2 Number of Carriers Required Back-off N BO_(N) . . . . . . iBO_(i) . . . . . . 1 BO₁

Waveforms with more carriers are associated with larger back-offs, sothat BO_(N)> . . . BO₂>BO_(i).

The maximum number of carriers that may be allocated to each activeterminal may then be determined based on the required transmit power andthe maximum transmit power for the terminal. In particular, thedifference between the maximum and required transmit powers for theterminal is first computed. This computed difference is then comparedagainst the required back-offs in the table. The largest requiredback-off (BO_(S)) that is less than or equal to this computed differenceis then identified, and the number of carriers S associated with thisBO_(S) is determined. The terminal may then be allocated any number ofcarriers less than or equal to S.

The maximum number of carriers that may be allocated to each activeterminal may be determined initially, as described above. The actualnumber of carriers to allocate to each terminal may then be determinedbased on any number of additional factors. Such factors may relate to(1) the amount of data to be transmitted, (2) fairness, (3) the priorityof the terminals, and so on. The specific number of carriers actuallyallocated to each terminal is equal to or less than the maximum numberof carriers that may be allocated. The specific carriers allocated toeach terminal may or may not be contiguous.

Many wireless communication systems are operated in frequency bands withspectral mask requirements that limit the amount of out-of-bandemissions. For these systems, which carriers to assign to each activeterminal may be selected such that out-of-band emissions is reduced orminimized to the extent possible.

FIG. 4 shows typical emission requirements for a typical radio frequency(RF) operating band. The operating band has spectral mask requirementsthat are characterized by a particular maximum in-band emission and aparticular maximum out-of-band emission. The maximum in-band emissionmay be specified, for example, by a particular per-MHz transmit powerconstraint. Similarly, the maximum out-of-band emission may be specifiedby a specific per-MHz transmit power constraint below frequency f₁ andabove frequency f₂.

Power amplifiers are typically designed such that they are linear at lowto medium output power levels and become more non-linear at higheroutput power levels. Thus, when a power amplifier is operated at a highoutput power level, a higher level of non-linearity in the poweramplifier can cause intermodulation distortion that falls outside thesignal band. The amount of distortion is dependent on the specificdesign of the power amplifier and the output power level. If thenon-linearity and/or output power level is sufficiently high, then theresultant distortion can exceed the specified maximum out-of-bandemission requirement.

In another aspect, the specific carriers to assign to the activeterminals are determined by their required transmit powers. A terminalwith a higher path loss to the access point (e.g., a terminal locatednear the edge of the coverage area) and/or a higher required receivedsignal quality needs to transmit at a higher power level to achieve therequired received signal quality at the access point. This terminal isthus more likely to generate a high level of intermodulation distortion.The terminal may then be assigned with carriers near the middle of theoperating band so that the distortion may fall within the operatingband. The higher level of distortion from this terminal may causeadditional interference to other carriers, so the transmit powers forthese carriers may be increased accordingly to account for the higherlevel of interference.

Conversely, a terminal with a smaller path loss to the access point(e.g., a terminal located near the access point) and/or a lower requiredreceived signal quality can transmit at a lower power level and stillachieve the required received signal quality at the access point. Thisterminal is thus likely to generate a low level of intermodulationdistortion. The terminal may then be assigned with carriers near theedges of the operating band since the distortion will likely be belowthe specified maximum out-of-band emission requirement. The specificcarriers allocated to each terminal may be located within a particularportion of the operating band but need not be contiguous.

FIG. 4 also shows the assignment of carriers to active terminals in amanner to reduce out-of-band emissions. A group of carriers 410 near themiddle of the operating band may be assigned to a terminal that needs totransmit at a high output power level. Two groups of carriers 412 and414 near the edges of the operating band may be assigned to the same ordifferent terminals that can transmit at a low output power level. Theuplink transmissions on these groups of carriers are from multipleterminals. However, these uplink transmissions are superimposed in thesame plot in FIG. 4 for clarity.

In one carrier assignment scheme, carriers are assigned to activeterminals based on their required transmit powers. For a giventransmission interval, the number of carriers to allocate to each activeterminal is first determined (e.g., based on the required transmit powerof the active terminal and possibly other factors as described above).The active terminals may be associated with different required transmitpowers. The group of carriers to assign to the active terminal with thelargest required transmit power is then selected to be near the middleof the operating, the group of carriers for the active terminal with thenext largest required transmit power is selected to be those closest tothe middle of the operating band, and so on, and the group of carriersfor the active terminal with the lowest required transmit power is thenselected to be toward the edges of the operating band. This carrierassignment scheme can reduce out-of-band emissions to the extentpossible.

In another carrier assignment scheme, each usable carrier is associatedwith a respective threshold power level, and the carriers are assignedto active terminals based on the threshold power levels and the requiredtransmit powers for the terminals. In particular, a given carrier may beassigned to a terminal if the required transmit power is equal to orless than the threshold power level. The carriers near the middle of theoperating band may be associated with higher threshold power levels, andthose toward the edges of the band may be associated with lowerthreshold power levels. These threshold power levels may be selectedsuch that the specified out-of-band emissions can be met for a givenmulti-carrier modulation scheme. Thus, a terminal located near the edgeof the coverage area and having a higher required transmit power mayonly be assigned carriers near the middle of the operating band, whereasa terminal with a lower required transmit power may be assigned carriersanywhere within the operating band.

The carriers may also be assigned to the active terminals in some othermanners to reduce out-of-band emissions, and this is within the scope ofthe invention. Moreover, the carrier assignment techniques describedherein may be used alone or in combination with the carrier allocationtechniques described above.

FIG. 5 shows a flow diagram of an embodiment of a process 500 toallocate and assign carriers to active terminals. Initially, pertinentinformation regarding the transmit power of each terminal to bescheduled for data transmission is obtained (step 512). In anembodiment, the required and maximum transmit powers for each terminalare obtained. The required transmit power for each terminal may be sentby the terminal or obtained based on some other means. The maximumtransmit power for each terminal may be sent by the terminal, known apriori, or obtained based on some other means. In another embodiment,the difference between the maximum and required transmit powers for eachterminal is obtained. In yet another embodiment, the maximum transmitpower and the initial transmit power for each terminal may be obtained(e.g., during registration), and the required transmit power for theterminal may thereafter be estimated based on the initial transmit powerand an accumulation of all power control commands sent to the terminal.The pertinent transmit power information may thus be provided in variousforms.

The maximum number of carriers that may be allocated to each terminal isthen determined based on the transmit power information (e.g., based onthe required and maximum transmit powers) (step 514). This may beachieved by using various schemes such as the two carrier allocationschemes described above. A specific number of carriers is then allocatedto each terminal based on (1) the maximum number of carriers that may beallocated to the terminal, (2) the total number of carriers availablefor allocation to all terminals, and (3) any number of other factors(step 516). The number of carriers allocated to each terminal is boundedby the maximum number that may be allocated. Moreover, the sum of allcarriers allocated to the terminals is bounded by the total number ofcarriers available for allocation.

Specific carriers are then assigned to each terminal in a manner suchthat the amount of out-of-band emissions may be reduced or minimized(step 518). This may be achieved by using various schemes such as thetwo carrier assignment schemes described above. The assigned carriersfor each terminal may then be signaled to the terminal via a carrierassignment. Each scheduled terminal would then transmit using thespecific assigned carriers and for the scheduled time period.

FIG. 6 shows a block diagram of an embodiment of an access point 110 xand two terminals 120 x and 120 y in multiple-access multi-carriercommunication system 100.

On the downlink, at access point 110 x, a transmit (TX) data processor614 receives traffic data (i.e., information bits) from a data source612 and signaling and other information from a controller 620 and ascheduler 630. For example, controller 620 may provide power control(PC) commands that are used to adjust the transmit power of the activeterminals, and scheduler 630 may provide assignments of carriers for theterminals. These various types of data may be sent on differenttransport channels. TX data processor 614 encodes and modulates thereceived data using multi-carrier modulation (e.g., OFDM) to providemodulated data (e.g., OFDM symbols). A transmitter unit (TMTR) 616 thenprocesses the modulated data to generate a downlink modulated signalthat is then transmitted from an antenna 618.

At each of terminals 120 x and 120 y, the transmitted downlink modulatedsignal is received by an antenna 652 and provided to a receiver unit(RCVR) 654. Receiver unit 654 processes and digitizes the receivedsignal to provide samples. A received (RX) data processor 656 thendemodulates and decodes the samples to provide decoded data, which mayinclude recovered traffic data, messages, signaling, and so on. Thetraffic data may be provided to a data sink 658, and the carrierassignment and PC commands sent for the terminal are provided to acontroller 660.

Controller 660 directs data transmission on the uplink using thespecific carriers that have been assigned to the terminal and indicatedin the received carrier assignment. Controller 660 further adjusts thetransmit power used for the uplink transmissions based on the receivedPC commands.

For the uplink, at each active terminal 120, a TX data processor 674receives traffic data from a data source 672 and signaling and otherinformation from controller 660. For example, controller 660 may provideinformation indicative of the required transmit power, the maximumtransmit power, or the difference between the maximum and requiredtransmit powers for the terminal. The various types of data are codedand modulated by TX data processor 674 using the assigned carriers andfurther processed by a transmitter unit 676 to generate an uplinkmodulated signal that is then transmitted from antenna 652.

At access point 110 x, the transmitted uplink modulated signals from theterminals are received by antenna 618, processed by a receiver unit 632,and demodulated and decoded by an RX data processor 634. Receiver unit632 may estimate the received signal quality (e.g., the receivedsignal-to-noise ratio (SNR)) for each terminal and provide thisinformation to controller 620. Controller 620 may then derive the PCcommands for each terminal such that the received signal quality for theterminal is maintained within an acceptable range. RX data processor 634provides the recovered feedback information (e.g., the required transmitpower) for each terminal to controller 620 and scheduler 630.

Scheduler 630 uses the feedback information to perform a number offunctions such as (1) selecting a set of terminals for data transmissionon the uplink and (2) assigning carriers to the selected terminals. Thecarrier assignments for the scheduled terminals are then transmitted onthe downlink for these terminals.

For clarity, the techniques for managing PAPR have been describedspecifically for the uplink in an OFDMA system. These techniques mayalso be used for downlink transmission from the access point to theterminals. In one downlink transmission scheme, OFDMA is used for thedownlink similar to the uplink, and carrier multiplexing may be used totransmit data to multiple terminals on the downlink simultaneouslywithin a given time interval. In another downlink transmission scheme,data is transmitted to one terminal at a time in a time divisionmultiplex (TDM) manner. For both downlink transmission schemes, thenumber of carriers to allocate to each terminal and the specificcarriers to assign to each terminal may be determined as described abovebased on the required transmit power for the terminal. For the OFDMAdownlink transmission scheme, the available carriers may be assigned tomultiple terminals such that the PAPR of the downlink signal for allscheduled terminals is maintained within a particular target. For theTDM-OFDM downlink transmission scheme, the number of carriers assignedto the terminal being served may be selected such that the PAPR of thedownlink signal to this terminal is also maintained within the target.Data for each scheduled terminal may then be transmitted using thespecific assigned carriers and at the required transmit power for theterminal.

For OFDM, the data to be transmitted on each carrier is first modulated(i.e., symbol mapped) using a particular modulation scheme selected foruse for that carrier to provide one modulation symbol for each symbolperiod. The modulation symbols for each terminal are then scaled toachieve the required transmit power for the terminal. The unusedcarriers are provided with signal values of zero. For each symbolperiod, M scaled symbols for M usable carriers and N−M zeros for theunused carriers are transformed to the time domain using an inverse fastFourier transform (IFFT) to obtain a “transformed” symbol that includesN time-domain samples. To combat intersymbol interference caused byfrequency selective fading (which results from a multipath channel), aportion of each transformed symbol may be repeated to form acorresponding OFDM symbol. OFDM symbols generated in this manner fordifferent symbol periods are then processed to generate the downlinkmodulated signal that is transmitted to the terminals.

For the downlink, if the M usable carriers are all transmitted at thesame power level, then the PAPR of the OFDM waveform can be large.However, by assigning more carriers to terminals with lower requiredtransmit powers and fewer carriers to terminals with higher requiredtransmit powers, the PAPR of the waveform will be smaller. This wouldthen allow the power amplifier at the access point to be operated with asmaller back-off and at a higher output power level. This may in turnallow higher data rates to be used for one or more of the terminals.

The techniques described herein for managing PAPR for multi-carriermodulation may be implemented by various means. For example, thesetechniques may be implemented in hardware, software, or a combinationthereof. For a hardware implementation, the elements used to implementthe techniques at each of the access point and the terminal may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, other electronic units designed to perform thefunctions described herein, or a combination thereof.

For a software implementation, the techniques described herein may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory unit (e.g., memory units 622 and 662 in FIG. 6) and executedby a processor (e.g., controllers 620 and 660 and scheduler 630). Thememory unit may be implemented within the processor or external to theprocessor, in which case it can be communicatively coupled to theprocessor via various means as is known in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of managing peak-to-average power ratio(PAPR) for multi-carrier modulation in a wireless multi-carriercommunication system, comprising: determining a maximum number ofcarriers that may be allocated to each terminal in a plurality ofterminals based on a required transmit power for the terminal; computinga difference between a maximum transit power for the terminal and therequired transmit power for the terminal, wherein determining themaximum number of carriers that may be allocated to the terminalcomprises comparing the computed difference against required back-offvalues for different numbers of carries, and selecting from thedifferent number of carriers a number of carriers associates with alargest required back-off value that is smaller than the computeddifference; and allocating to each terminal a specific number ofcarriers that is less than or equal to the maximum number of carriersdetermined for the terminal.
 2. The method of claim 1, furthercomprising: assigning specific carriers to each terminal in a manner toreduce out-of-band emissions.
 3. The method of claim 2, wherein aterminal associated with a higher required transmit power is assignedcarriers located near the middle of an operating band and a terminalassociated with a lower required transmit power is assigned carrierslocated near an edge of the operating band.
 4. The method of claim 2,wherein the assigning specific carriers to each terminal comprises:determining a set of carriers that can be assigned to the terminal basedon the required transmit power for the terminal, and selecting thespecific carriers to assign to the terminal from the set of carriers. 5.The method of claim 1, further comprising: transmitting data for eachterminal on the specific number of carriers allocated to the terminaland at the required transmit power for the terminal.
 6. The method ofclaim 1, further comprising: receiving a data transmission from eachterminal on the specific number of carriers allocated to the terminal.7. The method of claim 1, wherein the specific number of carriersallocated to each terminal is further based on a total number ofcarriers available for allocation to the plurality of terminals.
 8. Themethod of claim 1, wherein the specific number of carriers allocated toeach terminal is further based on at least one additional factor.
 9. Themethod of claim 8, wherein the at least one additional factor includes afactor that relates to amount of data to transmit.
 10. The method ofclaim 1, wherein the required transmit power for each terminal isdetermined based on a required received signal quality for the terminal.11. The method of claim 1, wherein the required transmit power for eachterminal is determined based on a power control loop maintained for theterminal.
 12. The method of claim 1, wherein the wireless communicationsystem implements orthogonal frequency division multiplexing (OFDM). 13.A method of allocating carriers to manage peak-to-average power ratio(PAPR) in a wireless communication system that implements orthogonalfrequency division multiplexing (OFDM), comprising: determining amaximum number of carriers that may be allocated to each terminal in aplurality of terminals based on a required transmit power for theterminal; computing a difference between a maximum transit power for theterminal and the required transmit power for the terminal, wherein thedetermining the maximum number of carriers that may be allocated to theterminal is based on the computed difference; allocating to eachterminal a specific number of carriers that is less than or equal to themaximum number of carriers determined for the terminal; and assigningspecific carriers to each terminal in a manner to reduce out-of-bandemissions, wherein the determining comprises comparing the computeddifference against required back-offs for different numbers of carriers,and wherein the maximum number of carriers that may be allocated to theterminal is equal to the number of carriers associated with a largestrequired back-off that is smaller than the computed difference.
 14. Anapparatus in a wireless multi-carrier communication system, comprising:means for determining a maximum number of carriers that may be allocatedto each terminal in a plurality of terminals based on a requiredtransmit power for the terminal; means for computing a differencebetween a maximum transit power for the terminal and the requiredtransmit power for the terminal, wherein the means for determining themaximum number of carriers that may be allocated to the terminal isbased on the computed difference; and means for allocating to eachterminal a specific number of carriers that is less than or equal to themaximum number of carriers determined for the terminal; wherein themeans for determining for each terminal further comprises means forcomparing the computed difference against required back-offs fordifferent numbers of carriers, and wherein the maximum number ofcarriers that may be allocated to the terminal is equal to the number ofcarriers associated with a largest required back-off that is smallerthan the computed difference.
 15. The apparatus of claim 14, furthercomprising: means for assigning specific carriers to each terminal in amanner to reduce out-of-band emissions.
 16. The apparatus of claim 14,further comprising: means for receiving a data transmission from eachterminal on the specific number of carriers allocated to the terminal .17. The apparatus of claim 14, further comprising: means fortransmitting data to each terminal on the specific number of carriersallocated to the terminal and at the required transmit power for theterminal.
 18. An access point in a wireless multi-carrier communicationsystem, comprising: a scheduler operative to determine a maximum numberof carriers that may be allocated to each terminal in a plurality ofterminals based on a required transmit power for the terminal, tocompute a difference between a maximum transmit power for the terminaland the required transmit power for the terminal, wherein the scheduleroperative to determine the maximum number of carriers that may beallocated to the terminal is operative to compare the computeddifference against required back-off values for different numbers ofcarries, and select from the difference number of carriers a number ofcarriers associated with a largest required back-off value that issmaller than the computed difference and wherein the scheduler isfurther operative to allocate to each terminal a specific number ofcarriers that is less than or equal to the maximum number of carriersdetermined for the terminal, and provide carrier assignments indicativeof the specific number of carriers allocated to each terminal; and atransmit data processor operative to process the carrier assignments fortransmission to the plurality of terminals.
 19. A method of transmittingdata using multi-carrier modulation in a wireless communication system,comprising: receiving an allocation of a specific number of carriers tobe used by a terminal for data transmission, wherein the specific numberof carriers is less than or equal to a maximum number of carriers thatmay be used for data transmission, wherein the maximum number ofcarriers is determined based on a difference between a maximum transmitpower for the terminal and a required transmit power for the terminal,and wherein the maximum number of carriers allocated to the terminal isbased on a comparison of the computed difference against requiredback-off values for different numbers of carriers, and a number ofcarriers associated with a largest required back-off that is smallerthan the computed difference being selected from the different number ofcarriers; and transmitting data using the specific number of carriers.20. The method of claim 19, wherein data is transmitted on specificcarriers that have been assigned to reduce out-of-band emissions.
 21. Anapparatus in a wireless multi-carrier communication system, comprising:means for receiving an allocation of a specific number of carriers to beused by a terminal for data transmission, wherein the specific number ofcarriers is less than or equal to a maximum number of carriers that maybe used for data transmission, wherein the maximum number of carriers isdetermined based on a difference between a maximum transmit power forthe terminal and a required transmit power for the terminal, wherein themaximum number of carriers allocated to the terminal is based on acomparison of the computed difference against required back-off valuesfor different numbers of carriers, and a number of carriers associatedwith a largest required back-off that is smaller than the computeddifference being selected from the different number of carriers; andmeans for transmitting data using the specific number of carriers. 22.The method of claim 1, wherein the required back-offs associated withthe numbers of carriers are determined based upon a lookup table. 23.The method of claim 1, wherein a larger back-off is associated withlarger numbers of carriers, and a smaller back-off is associated withfewer numbers of carriers.
 24. A non-transitory computer-readablemedium, comprising: code for causing a computer to: receive anallocation of a specific number of carriers to be used by a terminal fordata transmission, wherein the specific number of carriers is less thanor equal to a maximum number of carriers that may be used for datatransmission, wherein the maximum number of carriers is determined basedon a difference between a maximum transmit power for the terminal and arequired transmit power for the terminal, and wherein the maximum numberof carriers allocated to the terminal is based on a comparison of thecomputed difference against required back-off values for differentnumbers of carriers, and a number of carriers associated with a largestrequired back-off that is smaller than the computed difference beingselected from the different number of carriers; and transmit data usingthe specific number of carriers.
 25. A non-transitory computer-readablemedium, comprising: code for causing a computer to: determine a maximumnumber of carriers that may be allocated to each terminal in a pluralityof terminals based on a required transmit power for the terminal;compute a difference between a maximum transmit power for the terminaland the required transmit power for the terminal, wherein the code todetermine the maximum number of carriers that may be allocated to theterminal comprises code to compare the computed difference againstrequired back-offs for different numbers of carriers, wherein a numberof carriers associated with a largest required back-off that is smallerthan the computed difference is selected from the different number ofcarriers; and allocate to each terminal a specific number of carriersthat is less than or equal to the maximum number of carriers determinedfor the terminal.
 26. An apparatus in a wireless multi-carriercommunication system, comprising: a receive data processor operative toreceive an allocation of a specific number of carriers to be used by aterminal for data transmission, wherein the specific number of carriersis less than or equal to a maximum number of carriers that may be usedfor data transmission, wherein the maximum number of carriers isdetermined based on a difference between a maximum transmit power forthe terminal and a required transmit power for the terminal, and whereinthe maximum number of carriers allocated to the terminal is based on acomparison of the computed difference against required back-off valuesfor different numbers of carriers, and a number of carriers associatedwith a largest required back-off that is smaller than the computeddifference being selected from the different number of carriers; and atransmit data processor operative to transmit data using the specificnumber of carriers.
 27. The method of claim 1, wherein required transmitpower for the terminal comprises an estimated required transmit powerfor the terminal.
 28. The method of claim 1, wherein required transmitpower for the terminal is based on information from a controller. 29.The method of claim 1, further comprising obtaining the requiredtransmit terminal for each terminal in the plurality of terminals.